Simpson joined the editorial staff of the OED in 1976 to work on the Supplement to the OED. He was Co-Editor of the Second Edition of the OED, published in 1989, and in 1993 was appointed Chief Editor, a position he held until his retirement in October 2013. As Chief Editor, he led the first comprehensive revision of the OED and oversaw the introduction of its online version.[1][2]

Simpson is a member of the English Faculty at the University of Oxford, an Emeritus Fellow of Kellogg College, Oxford, and a member of the Philological Society, where the idea of the Dictionary was first proposed in the 1850s. He is a founder member of the European Federation of National Institutions for Language[3] and has been a member of its Executive Committee since 2003. He has acted as an adviser to a number of other national dictionaries, including the Opera del Vocabolario Italiano and the Australian National Dictionary.